Diode having vertical structure

ABSTRACT

A light emitting diode includes a conductive layer, an n-GaN layer on the conductive layer, an active layer on the n-GaN layer, a p-GaN layer on the active layer, and a p-electrode on the p-GaN layer. The conductive layer is an n-electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit of, U.S.application Ser. No. 13/216,531, filed Aug. 24, 2011 (now U.S. Pat. No.8,592,846), which is a continuation of Ser. No. 12/941,627, filed Nov.8, 2010 (now U.S. Pat. No. 8,008,681), which is a continuation of Ser.No. 12/840,840, filed Jul. 21, 2010 (now U.S. Pat. No. 7,915,632), whichis a continuation of Ser. No. 12/654,894, filed Jan. 7, 2010 (now U.S.Pat. No. 7,863,638), which is a continuation of Ser. No. 11/593,470,filed Nov. 7, 2006 (now U.S. Pat. No. 7,821,021), which is acontinuation of Ser. No. 09/983,994, filed Oct. 26, 2001 (now U.S. Pat.No. 7,148,520), all of which are hereby incorporated by reference hereinin their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to diodes, and more particularly, to avertical structure light emitting diode (LED). Although the presentinvention is discussed with reference to a GaN-based light emittingdiode, the present invention can be used for various types of lightemitting diodes, and can be further used in a wide range of applicationsincluding, for example, other types of diodes such as laser diodes

2. Discussion of the Related Art

Light emitting diodes, commonly referred to as “LEDs,” are semiconductordevices which convert electrical power into emitted light. It is wellknown in the art that when electrons transition between their allowedenergy levels in atoms or molecules, these transitions are alwaysaccompanied by a gain or loss of specific quanta of energy. In a lightemitting diode, the generation or injection of a current of eitherelectrons or holes across the diode junction encourages such electronictransitions, which in turn result in either vibrational energy or light,or both. As is further known in the art, the color of light that can beproduced by a light emitting diode is generally limited by thecharacteristics of the semiconductor material, most significantly by theband-gap which represents the energy level differences between thevalence band and conduction band of individual atoms.

Gallium-Nitride (GaN) has recently gained much attention fromresearchers in the field of LEDs, since its material characteristics ofa wide and direct band gap are suitable for making a blue LED, which hasbeen considered the most difficult one to fabricate among other red andgreen LEDs

Accordingly, GaN-based opto-electronic device technology has rapidlyevolved from the realm of device research and development to commercialreality, since these devices have been introduced in the market in 1994.The efficiency of GaN light emitting diodes, for example, has surpassedthat of incandescent lighting, and is now comparable with that offluorescent lighting.

The market growth for GaN-based devices has been far exceeding than theindustrial market prediction every year. Despite of such rapid speed ofthe development, it is still too expensive to realize a full colordisplay with GaN-based devices. This is because the manufacturing costof blue LEDs, which are essential to realizing a full color display, ishigh compared with the other visible LEDs. The wafer size for makingblue LEDs is limited to 2 inches, and the process of growing a GaNepitaxial layer is more difficult than other semiconductor materials.Therefore, it is crucial that developments of mass productiontechnology, without sacrificing performance, are the main issue inreducing the manufacturing costs of blue LEDs, thereby to utilize fullcolor displays using GaN LEDs at an inexpensive price, the efficiency ofwhich is far better than currently available

In general, GaN-based LEDs are fabricated with a lateral structure usinga sapphire substrate, since sapphire is the material that makes the GaNepitaxial layer grow with fewer defects than other materials as asubstrate. Since sapphire is an electrical insulator, the lateral typeLEDs having both n and p metal contacts on the topside is inevitable toinject current flows in the MQW layer.

FIG. 1 schematically illustrates conventional lateral type LED device.Referring to FIG. 1, the convention lateral type LED includes asubstrate 100, such as sapphire. A buffer layer 120, which is optionaland is made of, for example, gallium nitride (GaN), is formed on thesubstrate 100. An n-type GaN layer 140 is formed on the buffer layer120. An active layer such as a multiple quantum well (MQW) layer 160 ofaluminum-indium-gallium-nitride (AlInGaN), for example, is formed on then-type GaN layer 140. A p-type GaN layer 180 is formed on the activelayer 160. A transparent conductive layer 220 is formed on the p-GaNlayer 180. The transparent conductive layer 220 may be made of anysuitable material including, for example, Ni/Au or indium-tin-oxide(ITO). A p-type electrode 240 is then formed on one side of thetransparent conductive layer 220. The p-type electrode 240 may be madeof any suitable material including, for example, Ni/Au, Pd/Au, Pd/Ni andPt. A pad 260 is formed on the p-type electrode 240. The pad may be madeof any suitable material including, for example, Au. The transparentconductive layer 220, the p-GaN layer 180, the active layer 160 and then-GaN layer 140 are all etched at one portion to form an n-electrode 250and pad 270.

Since sapphire is an insulator, the n-GaN layer should be exposed toform an n-metal contact. A dry-etching method is generally used, sinceGaN is not etched by a chemical etching method. This is a significantdisadvantage since additional lithography and stripping processes arerequired. In addition, plasma damages on the GaN surface are oftensustained during a dry-etch process. Moreover, the lateral devicestructure requires a large device dimension since two metal contactsneed to be formed on top of the device. Furthermore, the lateralstructure device is vulnerable to static electricity because two metalelectrodes are positioned close each other. Thus, the lateral structureGaN based LEDs may not be suitable for high voltage applications, suchas traffic indicators and signal lights.

Currently, a vertical structure of GaN-based LEDs is fabricated by CreeResearch Inc. using a silicon carbide (SiC) substrate. Due to the highmanufacturing cost, however, the LEDs with SiC substrate are notsuitable for mass production. In addition, SiC is known in the art to bevery sensitive to hydrogen atoms, which exist during the epitaxialgrowth of GaN layer by metal organic chemical vapor deposition (MOCVD)method, which is the most common way of growing GaN epitaxial layersconsidering the epitaxial film quality. An additional process called“surface treatment” is necessary in order to grow high quality GaN-basedepitaxial films. Furthermore, the GaN based LEDs with a SiC substraterequires an additional conductive buffer layer on the SiC substratebefore growing the GaN epitaxial layer, which is not necessary forlateral structure devices.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method offabricating simple vertical structure LEDs for mass production thatsubstantially obviates one or more of the problems due to limitationsand disadvantages of the related art.

An advantage of the present invention is to increase the number of LEDdevices fabricated within a limited wafer area.

Another advantage of the present invention is LED devices having asimplified fabrication process steps.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method ofmaking light emitting diodes comprises forming a buffer GaN layer byvapor phase epitaxy (VPE) on a sapphire substrate; forming an n-GaNepitaxial layer by MOCVD on the buffer GaN; forming a multi-quantum-well(MQW) layer on the n-GaN epitaxial layer; forming a p-GaN layer on theMQW layer by MOCVD; a step of separating the sapphire substrate fromother layers; forming p and n metal contacts; forming a metaltransparent contact on the side of the p-GaN layer; and forming a metalpad on the p-GaN layer.

In another aspect, a method of making light emitting diodes comprisesforming, a buffer GaN layer by VPE on a sapphire substrate; forming anundoped GaN layer by VPE on the buffer GaN layer; forming an n-GaN layerby VPE on the undoped GaN layer; forming a n-GaN epitaxial layer byMOCVD on the n-GaN grown by VPE; forming a MQW layer on the n-GaNepitaxial layer; forming a p-GaN layer on the MQW layer by MOCVD; a stepof separating the sapphire substrate from other layers; forming p and nmetal contacts; forming a metal transparent contact on the p-GaN layer;and forming a metal pad on the p-GaN layer.

In another aspect, a method of making light emitting diodes comprisesforming a buffer GaN layer by VPE on a sapphire substrate; forming ann-GaN epitaxial layer by MOCVD on the buffer GaN layer; forming a MQWlayer on the n-GaN epitaxial layer; forming a p-AlGaN cladding layer onthe MQW layer by MOCVD; forming a p-GaN conducting layer on the p-AlGaNlayer by MOCVD; a step of separating the sapphire substrate from otherlayers; forming p and n metal contacts; forming a metal transparentcontact on the p-GaN layer; and forming a metal pad on the p-GaN layer.

In a further aspect, a method of making light emitting diodes comprisesforming a buffer GaN layer by VPE on a sapphire substrate; forming anundoped GaN layer by VPE on the buffer GaN layer; forming an n-GaN layerby VPE on the undoped GaN layer; forming a n-GaN epitaxial layer byMOCVD on the n-GaN grown by VPE; forming a MQW layer on the n-GaNepitaxial layer; forming a p-AlGaN cladding layer on the MQW layer byMOCVD; forming a p-GaN conducting layer on the p-AlGaN layer by MOCVD; astep of separating the sapphire substrate from other layers; forming pand n metal contacts; forming a metal transparent contact on the p-GaNlayer; and forming a metal pad on the p-GaN layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 shows a conventional lateral structure LED;

FIG. 2 shows a vertical structure LED according one embodiment of thepresent invention;

FIGS. 3-8 show the manufacturing steps for forming the light emittingdiode according to the present invention; and

FIG. 9 shows another embodiment of the vertical structure LED of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present invention, examplesof which are illustrated in the accompanying drawings.

FIG. 2 shows a vertical structure light emitting diode in accordancewith one embodiment of the present invention. Referring to FIG. 2, thevertical LED includes an n-contact 500. A buffer layer made of GaN 105is on the n-contact 500. An n-GaN layer 140 is on the buffer layer 105.An active layer 160 made of, for example, a multiple quantum well (MQW)layer including AlInGaN is on the n-GaN layer 140. A p-GaN layer 180 ison the active layer 160. A p-contact layer 220 is on the p-GaN layer180. A p-electrode 240 and a pad 260 are formed on the p-contact layer220.

In the LED shown in FIG. 2, the n-contact 500 may serve two functions.First, the n-contact 500 serves an electrical function as a conductivematerial. Second, the n-contact 500 can also serve to reflect photonsemitted from the active layer 160 to the n-contact 500. This increasesthe brightness of the LED since photons that would otherwise be absorbedor wasted in some other manner would reflect off of the n-contact 500and emit light. A material having good reflective characteristics suchas that of a mirror can be used as the n-contact 500. One such exampleis a polished aluminum layer. Such reflective characteristics aredescribed in more detail in a co-pending application entitled “DIODEHAVING HIGH BRIGHTNESS AND METHOD THEREOF” by Myung Cheol Yoo, filed onJul. 17, 2001 by the same assignee as the present application, theentirety of which contents is hereby incorporated by reference in thisapplication. The material for the n-contact 500 is discussed in detailbelow.

A benefit of this vertical structure LED of the present invention is thesignificant reduction in the size of the LED chip as compared to thelateral structure of the conventional LED. Due to its small chip size,significantly more chips can be formed on the same size wafer, such assapphire. Moreover, the number of process steps for forming the verticalstructure LED of the present invention is reduced, as discussed in moredetail below. FIGS. 3-8 schematically illustrate a process formanufacturing vertical structure GaN-based light emitting diodes (LEDs)according to the present invention. In order to fabricate GaN-basedLEDs, sapphire substrate has been generally used since sapphire is verystable and relatively inexpensive. The epitaxial layer quality of thevarious GaN layers grown on sapphire substrate is superior to othersubstrate materials due to their thermal stability and the similarcrystal structure of the GaN.

Referring to FIG. 3, a buffer layer 120, is formed on a transparentsubstrate 100, beneficially a sapphire substrate. The buffer layer 120,which eventually replaces the function of the sapphire substrate 100,may be formed as one, two or three layers. For example, the buffer layer120 may have only the n-GaN layer that is grown by VPE. For a two layerbuffer layer, a first layer of GaN layer 110 is grown on the sapphiresubstrate such as by VPE and a second layer of an n-GaN layer 120 isgrown on the GaN layer 110 such as by VPE. For a three layer bufferlayer, a first layer of GaN layer 110 is grown on the sapphire substratesuch as by VPE, a second layer of an undoped GaN layer 130 is grown onthe first layer of GaN layer 110 such as by VPE, and a third layer of ann-GaN layer 120 is grown on the undoped GaN layer 130 such as by VPE.

The GaN layer 110 may be formed to have a thickness in a range of about40-50 nm. The undoped GaN layer 130 may be formed to have a thickness ina range of about 30-40 μm. The n-GaN layer 120 may be formed to have athickness of about 1-2 μm. For n-GaN 120, silene gas SiH₄ may be used asthe n-type dopant.

Referring to FIG. 4, an n-type epitaxial layer such as n-GaN 140 isepitaxially grown on the buffer layer 120 by a metal organic chemicalvapor deposition (MOCVD) method. Beneficially, a chemical cleaning step(not shown in the figure) of the buffer layer 120 grown by VPE methodcan be added prior to growing the n-GaN layer 140 by MOCVD method inorder to obtain a good quality of the n-GaN epitaxial layer 140. In thisinstance, the n-GaN layer 140 was doped with silicon (Si) with a dopingconcentration of about 10¹⁷ cm⁻³ or greater.

Referring to FIG. 5, an active layer 160 such as an AlInGaN multiplequantum well (MQW) layer is formed by MOCVD method on the n-GaN layer140. The active layer 160 may be of any suitable structure including asingle quantum well layer or a double hetero structure. In thisinstance, the amount of indium (In) determines whether the diode takeson a green color or a blue color. For an LED with blue light, about 22%of indium may be used. For an LED with green light, about 40% of indiummay be used. The amount of indium used may be varied depending on thedesired wavelength of the blue or green color. Subsequently, a p-GaNlayer 180 is formed by MOCVD method using, for example, CP₂Mg as ap-type dopant on the active layer 160. In this instance, the p-GaN layer180 was doped with magnesium (Mg) with a doping concentration of about10¹⁷ cm⁻³ or greater.

Referring to FIG. 6A, the sapphire substrate 100 is separated from otherlayers preferably by a laser lift-off method. Other suitable techniquesmay be used to separate the sapphire substrate 100 from the otherlayers. The other layers include the buffer layer 120, n-GaN layer 140,active layer 160, and the p-GaN layer 180. By removing the sapphiresubstrate 100, which is an electrical insulator, from the device, ann-metal contact can be formed under the n-type GaN buffer layer 120,which is an electrical conductor.

Referring to FIG. 6B, after the substrate 100 is removed, the layersbelow the buffer layer 120 may be removed as well using, for example,dry etching. This step will expose the n-GaN buffer layer 120 that willbe electrically attached to the n-contact 500, as shown in FIG. 8.

Referring to FIG. 8, a transparent conductive layer 220 is formed on thep-GaN layer 180. The transparent conductive layer 220 may be made of anysuitable material including, for example, indium-tin-oxide (ITO). Ap-type electrode 240 is formed on the transparent conductive layer 220.An n-type electrode 500 is formed on the bottom of the buffer layer 120.The p-type electrode 240 may be made of any suitable material including,for example, Ni/Au, Pd/Au, Pd/Ni and Pt. The n-type electrode 500 may bemade of any suitable material including, for example, Ti/Al, Cr/Au andTi/Au. A pad 260 is formed on the p-type electrode 240. The pad 260 maybe made of any suitable material including, for example, Au. The pad 260may have a thickness of about 0.5 μm or higher. Unlike the p-typeelectrode 240, the n-type electrode 500 does not require a pad, althoughone can be used, if desired.

FIG. 9 shows an alternative embodiment in which a cladding layer 170 isformed between the p-GaN layer 180 and the active layer 160. Thecladding layer 170 is preferably formed with p-AlGaN by MOCVD methodusing CP₂Mg as a p-type dopant. The cladding layer 170 enhances theperformance of the LED device.

According to the present invention, there are many advantages comparedwith both conventional lateral and vertical GaN-based LEDs. Comparedwith the conventional lateral structure GaN-based LEDs, themanufacturing process according to the present invention increases thenumber of LED devices fabricated on a given wafer size, since there isno n-metal contact on top of the devices. The device dimension can bereduced, for example, from 250×250 μm to about 160×160 μm or smaller. Bynot having the n-metal contact above the substrate or on top of thedevice, according to the present invention, the manufacturing process issignificantly simplified. This is because additional photolithographyand etch processes are not required to form the n-metal contact andthere is no plasma damage which are often sustained on the n-GaN layerin the conventional lateral structure GaN-based LEDs. Furthermore, theLED devices fabricated according to the present invention are much moreimmune to static electricity, which makes the LED more suitable for highvoltage applications than conventional lateral structure LED devices.

In general, the deposition method of VPE is much simpler and requiresless time to grow epitaxial layers with certain thickness than thedeposition method of MOCVD. Therefore, the fabrication process is moresimplified and the process time is more reduced even compared with thoseof the conventional vertical GaN-based LEDs in that the manufacturingprocess according to the present invention does not require growingbuffer and n-GaN layers by MOCVD method. In total, the number ofmanufacturing steps is reduced, for example, from 28 steps with theconventional method to 15 steps with the method of the presentinvention. In addition, the manufacturing cost is reduced considerablycompared with the conventional vertical structure GaN-based LEDs, whichuse silicon carbide (SiC) as a substrate, which can be 10 times moreexpensive than that of a sapphire substrate. Moreover, the methodaccording to the present invention provides better metal adhesionbetween bonding pads and both n and p contacts than the conventionalvertical structure GaN-based LEDs.

With the present invention, mass production of GaN-based LEDs at aninexpensive cost is made possible without sacrificing or changing thedesired characteristics of the LEDs. Moreover, the vertical structure ofthe LED of the present invention, with an added feature of a reflectivebottom n-contact, enhances the brightness of the LED. This invention canbe applied not only to the current commercially available blue, green,red and white LEDs but also to other suitable devices.

Although the present invention has been described in detail withreference to GaN technology diodes, the present invention can easily beapplied to other types of diodes including red LEDs and laser diodesincluding Vertical Cavity Surface Emitting Lasers (VCSELs).

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the split or scope of the invention. Thus, it is intendedthat the present invention cover the modifications and variations ofthis invention provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A light emitting device comprising: an Al layer;an undoped GaN buffer layer on the Al layer; an n-GaN layer on theundoped GaN buffer layer; an active layer comprising a multiple quantumwell (MQW) layer on the n-GaN layer; a p-GaN layer on the active layer;and a transparent conductive layer comprising indium-tin-oxide (ITO) onthe p-GaN layer; wherein the undoped GaN buffer layer is thicker thanthe Al layer.
 2. The device according to claim 1, wherein a ratio of athickness of the Al layer to a thickness of the undoped GaN buffer layeris greater than 0.0075.
 3. The device according to claim 1, wherein then-GaN layer is doped with Si with a doping concentration of 10¹⁷ cm⁻³greater.
 4. The device according to claim 1, wherein the p-GaN layer isdoped with Mg with a doping concentration of 10¹⁷ cm⁻³ greater.
 5. Thedevice according to claim 1 further comprising a metal layer comprisingAu.
 6. The device according to claim 1, wherein the Al layer serves as areflective layer.
 7. The device according to claim 1 further comprisinga transparent substrate on the Al layer.
 8. The device according toclaim 1 further comprising a clad layer including AlGaN between theactive layer and the p-GaN layer.
 9. A light emitting device comprising:a metal layer comprising Au; an Al layer on the metal layer; an undopedGaN buffer layer on the Al layer; an n-GaN layer on the undoped GaNbuffer layer; an active layer comprising a multiple quantum well (MQW)layer on the n-GaN layer; a p-GaN layer on the active layer; and atransparent conductive layer comprising indium-tin-oxide (ITO) on thep-GaN layer; wherein a thickness of the undoped GaN buffer layer isthicker than a thickness of the Al layer.
 10. The device according toclaim 9, wherein a ratio of the thickness of the Al layer to thethickness of the undoped GaN buffer layer is greater than 0.0075. 11.The device according to claim 9, wherein the n-GaN layer is doped withSi with a doping concentration of 10¹⁷ cm⁻³ or greater.
 12. The deviceaccording to claim 9, wherein the p-GaN layer is doped with Mg with adoping concentration of 10¹⁷ cm⁻³ or greater.
 13. The device accordingto claim 9, wherein the Al layer serves as a reflective layer.
 14. Thedevice according to claim 9 further comprising a transparent substrateon the Al layer.
 15. The device according to claim 9 further comprisinga clad layer including AlGaN between the active layer and the p-GaNlayer.
 16. A light emitting device comprising: a metal layer comprisingAu; an Al layer on the metal layer; an undoped GaN buffer layer on theAl layer; an n-GaN layer on the undoped GaN buffer layer; an activelayer comprising a multiple quantum well (MQW) layer on the n-GaN layer;a p-GaN layer on the active layer; and a transparent conductive layercomprising indium-tin-oxide (ITO) on the p-GaN layer; wherein theundoped GaN buffer layer is thicker than the Al layer, wherein ratio ofa thickness of the Al layer to a thickness of the undoped GaN bufferlayer is greater than 0.0075, wherein the n-GaN layer is doped with Siwith a doping concentration of 10¹⁷ cm⁻³ or greater, and wherein thep-GaN layer is doped with Mg with a doping concentration of 10¹⁷ cm⁻³ orgreater.
 17. The device according to claim 16, wherein the Al layerserves as a reflective layer.
 18. The device according to claim 16further comprising a transparent substrate on the Al layer.
 19. Thedevice according to claim 18, wherein the Al layer covers a bottomsurface of the transparent substrate.
 20. The device according to claim16 further comprising a clad layer including AlGaN between the activelayer and the p-GaN layer.